The present invention relates to a method for minimizing surface roughening of metallic articles made of tungsten-containing alloys such as cemented carbides or high-speed steels. More particularly, it is concerned with a method for improving the resistance to surface roughening of tungsten-containing metallic article such as mill rolls and cutting tools. The invention also relates to a tungsten-containing metallic article having improved resistance to surface roughening and a method for retarding surface-roughening of tungsten-containing hot mill rolls during hot rolling.
Most cutting tools have tips made of tungsten-containing alloys which include high-speed steels and cemented tungsten carbide hard alloys. High-speed steels (also called high-speed tool steels) usually contain a considerable amount (generally 5-22% by weight) of W and one or more other alloying elements such as Mo, Cr, V, and Co. Cemented carbide alloys are composite alloys predominantly comprising tungsten carbide (WC) particles which are sintered with one or more binder metals such as Ni, Co, and the like. These two types of tungsten-containing alloys are both very expensive due to their high contents of expensive metals such as tungsten, nickel, and cobalt. Therefore, many alloy designs have been proposed with respect to tungsten-containing alloys for cutting tools in order to extend the lifetime of the tools and improve the cuttability thereof.
Mill rolls used in rolling of steel products are made of various materials. High-speed steels and cemented tungsten carbide hard alloys are mainly used in mill rolls for finish rolling and especially finish hot rolling for high-strength wire rods. Surface roughening of rolls is crucial particularly in finish rolling since surface gloss and smoothness are important properties for steel products and any surface roughness and flaws on the surface of finish rolls are printed onto rolled steel products. Therefore, it is important that rolls for finish rolling have a smooth roll surface.
In order to cool hot mill rolls, industrial water normally available in the factory has conventionally been used without treatment. However, as reported in Sumitomo Electric (a company journal of Sumitomo Electric Industry, Japan), No. 123, page 113, the rolls suffer surface roughening primarily due to corrosion in a relatively short period during hot rolling, resulting in a deterioration in surface appearance of rolled products.
In order to dress the roughened roll surface, the rolling operation must be interrupted after a short period, e.g., 2 hours and the surface-roughened rolls are detached for replacement by another set of rolls and subjected to a surface dressing procedure such as grinding and electrolytic polishing to restore the damaged roll surface to a smooth surface. This dressing procedure, which must be repeated frequently during hot rolling is a serious problem in a hot rolling operation since it leads to a significant decrease in operating efficiency. Such frequent dressing of rolls also results in a decreased lifetime of rolls made of a tungsten-containing alloy, which are much more expensive than conventional cast steel rolls, and therefore increases operating costs.